Diesel engines emit a low amount of hydrocarbons and carbon monoxide, but emit a relatively high amount of nitrogen oxides (NOx) and particulates (smoke). Among diesel exhaust products, smoke is solid or liquid particles contained in the gas generated via combustion or thermal decomposition processes of the engine, and is classified into black smoke, blue smoke, yellow smoke, and white smoke according to the visible floating components thereof. There may be many complex causes of emission of white smoke, and the white smoke includes condensed water, liquid fuel, or sulfide components in the exhaust gas.
The present applicant has disclosed a smoke reduction apparatus for preventing white smoke from being emitted from diesel engines in Korean Patent Application No. 2011-134404 (Dec. 14, 2011). According to this, it was confirmed that, in the smoke reduction apparatus including a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF) mounted at the rear end of a diesel engine, when the platinum/palladium weight ratio (Pt/Pd) of the diesel oxidation catalyst composition applied to the DOC is larger than 0 and smaller than 1 (0<Pt/Pd<1), the emission of white smoke generated during initial startup of the diesel engine, especially in winter, is significantly reduced.